Bulletin ISSN 1429-2335 3(151) 2000 of the Sea Institute

CONTENTS

Scientific papers

LEONARD EJSYMONT and KENNETH SHERMAN Poland and ’ Cooperation in Fisheries : A Multidecadal Retrospective ...... 3

MORGAN S. BUSBY, ANN C. MATARESE, DEBORAH M. BLOOD and MA£GORZATA KONIECZNA Advancements in Ichthyoplankton Taxonomy in the Northeastern Pacific Ocean and Bering Sea: Research Conducted by the Center 1965-1999 ...... 11

ARTHUR W. K ENDALL, JR. Status of Recruitment Studies of Northeast Pacific Fishes ...... 21

ALLYN B. POWELL, DONALD E. HOSS, MA£GORZATA KONIECZNA and LEONARD EJSYMONT Summary of Ichthyoplankton Research by the NOAA Beafort Laboratory in Florida Bay, Everglades National Park, Florida, USA ...... 43

JOANNE LYCZKOWSKI-SHULTZ, MA£GORZATA KONIECZNA and WILLIAM J. RICHARDS Occurrence of the Larvae of Beryciform Fishes in the Gulf of Mexico ...... 55

JACK W. JOSSI and JOSEPH KANE An Atlas of Seasonal Mean Abundances of the Common Zooplankton of the United States Northeast Continental Shelf Ecosystem ...... 67

KENNETH SHERMAN Marine Ecosystem Management of the Baltic and Other Regions ...... 89

BENJAMIN H. SHERMAN Marine Disease, Morbidity and Mortality,Toward a Baltic Sea Ecological Disturbance Information System ...... 101

PIOTR MARGOÑSKI Impact of Hydrological and Meteorological Conditions on the Spatial Distribution of Larval and Juvenile Smelt (Osmerus eperlanus) in the Vistula Lagoon (Southern Baltic Sea) 119 8. Footnotes should be marked with Arabic numerals INSTRUCTIONS in superscript ( ...1), and numbered in succession throughout the text, except for tables; footnote content FOR AUTHORS should be on separate sheets of paper. 9. Tables should supplement, not duplicate, data contained in the text or figures. Tables should be GENERAL INFORMATION numbered and each one should be on a separate sheet of The Bulletin of the Sea Fisheries Institute is a scientific paper. All tables must have titles; all references to them journal which accepts papers from all over the world. should be placed within the text. Each column in a table Foreign authors are requested to submit their papers in is supplied with a heading, explaining the content of the English, the research staff of the SFI in Polish and column. 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Status of Recruitment Studies of Northeast Pacific Fishes

Arthur W. Kendall, Jr. Alaska Fisheries Science Center, Seattle, WA 98115, USA

Abstract. Recruitment research is vital to , since year-class strength is the most significant biological variable affecting abundance of high latitude fish populations. Year- class strength is usually not well correlated with spawning stock size or environmental variables. Variable survival of early life stages (eggs and larvae) is critical in determining year-class size. The recruitment problem has a long history in fisheries research and its study continues today. The history of such studies in the temperate and subarctic northeast Pacific are traced back to work on Pacific halibut by W. F. Thompson and others. With the discovery in 1980 of a concentrated spawning aggregation of walleye pollock in Shelikof Strait, Gulf of Alaska, recruitment research found a focus that led to expanded studies there and in the Bering Sea. This research has led to annual forecasts of relative year-class strength that help guide management of pollock harvests. In the future, more emphasis needs to be placed on time-scales other than interannual and ecosystem responses such as system productivity, changes in species dominance, and system maturity. This emphasis will require complex long-term and multi-trophic level studies.

Keywords: walleye pollock, Bering Sea, Gulf of Alaska, year-class strength, climate change, FOCI,

INTRODUCTION

One of the most pervasive characteristics of marine fish populations is that they vary tremendously in abundance over time. At least ten-fold variations in abundance over a few years are common. This creates major problems for the industry and fisheries managers alike: what are the sources of these variations (how much is due to fishing and how much is due to natural variations?), and how can we manage fisheries in the face of them? It has long been realized that natural fluctuations in abundance are largely due to variations in the size of year classes caused by events that modify mortality of young stages from eggs through early juveniles (Smith 1994). Stock sizes are largely determined by the abundance of their component year classes. Although the goal of considerable research has been to understand the causes of fluctuations in year-class strength, this goal remains elusive, and variations in recruitment are still the major source of biological variability facing fisheries managers (Sissenwine 1984; Rothschild 1986). Since mortality of early, mainly planktonic stages is implicated, recruitment 22 ARTHUR W. K ENDALL, JR. studies involve the interaction of fish and their environment: physical and chemical, as well as biological. However, simple correlations of year-class strength with spawning potential of the population or environmental variables believed to be important to the survival of young stages are almost always ineffective predictors. Although several hypotheses have been developed to explain the recruitment process (e.g. variations in food production for first-feeding larvae, drift of later larvae in relation to nursery areas (Hjort 1926), timing of the spring bloom in relation to the hatching period of larvae (Cushing 1975), and turbulence and stratification and their effect on larval food (Lasker 1975), none alone has proven sufficient. In at least some cases, interannual differences in survival of juveniles may also be important (Sissenwine 1984). Density-dependent effects also must be important, but they have rarely been convincingly demonstrated (see Myers and Cadigan 1993). We are left with the conclusion that the recruitment process must involve the cumula- tive effects of many factors acting at different times during the early life histories of fishes. The most critical factors may vary with species, and even for one species, may not be the same every year. As a result of our difficulties in understanding the recruitment process, fisheries must be managed conservatively, assuming the majority of year classes will be weak. When recruit- ment is stronger, this results in setting harvest levels that are lower than necessary to maintain a population. If recruitment could be accurately predicted, fisheries harvest strategies could take better advantage of the occasional strong year class. Understanding longer term environ- mental changes (e.g. climate) and their impact on ecosystems including fished stocks could lead to significant changes in management strategies (Steele 1996). This paper sets the stage for and describes recruitment studies that have been conducted on marine fishes in the temperate and subarctic northeast Pacific Ocean and eastern Bering Sea. After examining the progress and status of our understanding of recruitment processes of fishes in this area, the potential future direction of such studies is outlined.

HISTORICAL BACKGROUND OF RECRUITMENT RESEARCH

Johan Hjort can be thought of as the father of recruitment studies in fisheries (Kendall and Duker 1998). Although initially a proponent of the widely accepted migration hypothesis to explain fluctuations in fish abundance, through studies of age structure of fish populations and the work of G.O. Sars (Sars 1877), Hjort realized that these fluctuations were mainly caused by variations in year-class strength: “The rich year classes appear to make their influence felt when still quite young; in other words, the numerical value of a year class is apparently determined at a very early stage, and continues in approximately the same relation to that of other year classes throughout the life of the individuals” (Hjort 1914). As mechanisms for establishing year-class strength, he suggested the importance of food for newly hatched larvae and drift to nursery areas: “As factors, or rather events which might be expected to determine the numerical value of a new year-class, I drew attention to the following two possibilities: (1) That those individuals which at the very moment of their being hatched did not suc- ceed in finding the very special food they wanted would die from hunger. That in other words the origin of a rich year-class would require the contemporary hatching of the eggs and the Status of Recruitment Studies ... 23 development of the special sort of plants or nauplii which the newly hatched larva needed for its nourishment. (2) That the young larvae might be carried far away out over the great depths of the Norwegian Sea, where they would not be able to return and reach the bottom on the continental shelf before the plankton in the waters died out during the autumn months of their first year of life” (Hjort 1926). Through his publications and work with ICES (from its beginning in 1902 until his death in 1948, at which time he was its president), Hjort’s insight into the causes of fluctuations in the abundance of fishes has stimulated fisheries scientists through the twentieth century. As a result of Hjort’s first hypothesis, the period when larvae are changing from getting nourishment from their yolk to when they must feed exogenously became known as the “critical period”, and it continues to be a central theme of recruitment research (see May 1974). In 1914, Hjort undertook a comprehensive investigation of Atlantic waters off the outer coast of Nova Scotia (Hjort 1919; Coker 1962). At the beginning of his survey of fisheries resources, he found himself without needed instruments, so he traveled to to borrow them from , who was studying the , plankton and fish of the Gulf of Maine (Hubbard 1993). During this visit, it is apparent that fluctuations in fish abundance were discussed (see Hubbard 1993). Certainly Hjort’s ideas are reflected in the research of two of Bigelow’s later students, Lionel Albert Walford and . Sette’s first published paper dealt with the Pacific . It mentioned interannual fluctuations in abundance, a theme that would pervade the rest of his career (Sette 1920). Two years after graduating from in 1922, Sette began his career as a Federal fisheries biologist in Washington, D.C. He was soon puzzling over variations in landings which led him to his doctoral work and a classic publication on the effect of the environment on larval survival, and ultimately year-class strength of Atlantic mackerel (Sette 1943a). Sette concluded, by measuring interannual changes in drift and mortality of cohorts of larvae, that increased mortality during the transition from yolk to exogenous food sources was not a major contributor to variations in mortality, but that variations in drift caused by winds seemed to be correlated with year-class strength. Thus, Sette rejected Hjort’s first hypothesis (the critical period) but concurred with his second hypothesis (drift). This was a pioneering study in that population estimates of larval growth and mortality rates were calculated. Sette conducted these studies in a laboratory on the Harvard University campus. During this period, from 1928-1937, Walford was studying recruitment of haddock at Harvard. Walford was a native and, like Sette, a Stanford graduate. Hjort’s influence on Walford’s work is clearly seen in the introduction to the paper on haddock recruitment that resulted from his thesis research, although he did not cite Hjort’s studies (Walford 1938). In 1937, Sette and Walford returned to California to lead Federal investigations into fluctuations in the sardine population and became organizers of what would become the Cali- fornia Cooperative Oceanic Fisheries Investigations (CalCOFI). Sette maintained that to man- age fisheries properly, the causes of fluctuations must be understood. Man’s impact through fishing could only be evaluated in the context of environmentally induced changes in abun- dance and distribution. He developed an exhaustive plan for studies that emphasized under- standing causes of mortality of young stages (Sette 1943b). CalCOFI has been the largest and most long-lasting of the fisheries oceanography studies in the United States (Scheiber 1990). From the beginning, the program had a broad emphasis: to study the sardine, its environment, and the effects of fishing on the species: “Ultimately we hope to be able to predict what the effect of the environment is on spawning success” (Lasker 1965). The impacts of large-scale 24 ARTHUR W. K ENDALL, JR. circulation variations on the California Current became appreciated, starting with the 1957-58 El Niño. However, understanding the causes of fluctuations in abundance of , includ- ing the role of man, continued to be an elusive goal (Marr 1960; Radovich 1960; Murphy 1961). Pacific coast fisheries matters were under the dominating influence of William F. Thompson when Sette and Walford returned to California to investigate the causes of the declines in sardine catches. Thompson advocated studying fisheries themselves rather than fish ecology to develop means for managing them. He felt that the abundance of fish stocks would be correlated with catch rates. Sette’s study plan for sardine research, which included ecological studies on all life history stages, as well as studies on the impact of the fisheries, was in stark contrast to the more narrowly focused research that would have been advocated by Thompson. Besides undertaking extensive studies of catch records and age composition, as well as conducting tagging studies, Thompson conducted pioneering studies of Pacific halibut early life history (Thompson and Van Cleve 1934). He thoroughly and accurately illustrated and described their eggs and larvae, and related their distribution to ocean currents and tempera- ture. He conducted these studies at a time when Pacific halibut populations were beginning to rebound from severe . The International Fisheries Commission had established regulations for the fishery in order to reduce the catch, which Thompson believed had reached levels where egg output was insufficient to replenish the stocks. He considered the impact of environmental factors on survival of eggs and larvae to be less important in determining year- class success than the number of eggs produced at the existent low population levels. He suggested annual monitoring of eggs in the plankton as a means of measuring population size as it increased due to fishing restrictions. There is evidence that Thompson and his colleagues knew of the broadly and ecologically based approach to fisheries issues seen in Sette’s plan, but that he felt with limited resources available, studying the fisheries themselves would be sufficient (Scheiber 1994). The continued successful fishery for Pacific halibut is raised as evidence of the value of Thompson’s management strategy of controlling fishing effort in response to variations in catch rates, and during his career accorded him considerable politi- cal clout and approval from the industry. However, his thesis that variations in abundance were the result of fishing pressure rather than environmental factors led to one of the most contentious debates in the history of fisheries science (the Thompson-Burkenroad Debate: see Skud 1975). As concerns over fluctuating and heavily fished stocks mounted around the country, dis- cussions were held in , California in 1968 among several Federal scientists involved in early life history studies to investigate expanding the type of work CalCOFI was doing. These discussions resulted in a nationwide program: MARMAP (Marine Resources Monitoring, As- sessment, and Prediction). Among its goals, this program intended to standardize collection of fish egg and larval data as well as environmental data and to determine seasonal and annual variability in biological and environmental components of the shelf ecosystem that influence the size of recruiting fish populations (Sibunka and Silverman 1984). Several factors con- spired to limit participation of scientists from the northeast Pacific in MARMAP: the lingering effects of the Thompson model for fisheries research, an emphasis on Pacific salmon research, and minimally exploited marine fish populations. Status of Recruitment Studies ... 25

NORTHEAST PACIFIC RECRUITMENT STUDIES

The Fish Fauna When compared with life histories of fishes in other areas, several life history features of northeast Pacific fishes stand out which impact their potential as subjects for recruitment studies. Few northeast Pacific fishes spawn pelagic eggs (Fig. 1), whereas in other areas this is the dominant pattern. This is partially because an inordinate portion of northeast Pacific fishes are scorpaeniforms (Table 1) and most of them produce demersal eggs (e.g. cottids, hexagrammids). Also, within the scorpaeniforms, another group – the speciose and abundant rockfishes (Sebastes) – are live-bearers, releasing late yolk-sac larvae. The absence of pelagic eggs constrains the recruitment research that can be done since sampling demersal eggs cannot be done with plankton nets, making their distribution and mortality difficult to assess. Furthermore, larvae from demersal eggs are usually larger and initially better developed than those from pelagic eggs, so starvation is not as much of a factor, and thus larval mortality rates are reduced. In the northeast Pacific, walleye pollock (Theragra chalcogramma) and the flatfishes are the most important harvested marine fishes with pelagic eggs. To date, marine recruitment research in the northeast Pacific has focused almost exclusively on the large, ecologically and economically important pollock populations.

Early Studies Besides considerable descriptive work on early life histories of fishes, and the work of Thompson on halibut, relatively little work was done on recruitment of marine fish of the northeast Pacific until the 1980s. An exception was work on Pacific hake, initiated out of Seattle; however, hake were found to spawn mainly off California (Bailey 1981), so this early life history work was discontinued. Also, Canadian researchers examined the early life history characteristics (egg development and survival in relation to temperature and salinity) of selected species and related them to survival and recruitment (see Alderdice 1985). Until the early 1980s, most fisheries in the northeast Pacific, except those for salmon, crab, and halibut, were rather undeveloped, and conducted mainly by foreign fleets, so there was little impetus for extensive recruitment research.

Gulf of Alaska Research The increase in the domestic groundfish industry following the discovery of a large spawning aggregation of walleye pollock in Shelikof Strait, Gulf of Alaska, in 1980 opened the way for an extensive recruitment study entitled FOCI (Fisheries-Oceanography Coordinated Investigations). The program goal of FOCI is to understand the relationship between variations in the physical and biological environment and year-class strength of economically important fish and shellfish in Alaska waters using field, laboratory, and modeling studies. In response to several adjacent strong year classes in the late 1970s, pollock in Shelikof Strait reached peak abundance in 1981, and a large fishery developed there on the prespawning fish (Megrey 1989; 1990). This population seemed to be marked by large variations in year-class strength, which made it attractive for FOCI recruitment studies. FOCI studies on this population have been funded since 1985 and primarily involve a collaboration of National Oceanic and Atmospheric Administration (NOAA) physical scientists from the Pacific Marine Environmental Laboratory 26 ARTHUR W. K ENDALL, JR. Fig. 1. Representative early life history modes of northeast Pacific subarctic fishes. From Kendall 1981. Status of Recruitment Studies ... 27 (1989). (1989). et al. et Pacific are from Matarese Matarese from are Pacific . (1989). (1989). . et al et Worldwide Worldwide Pacific NE order by species species marine % 738 6 1.6 10 3.2 1.4

1 Families

World Pacific NE Marine Freshwater Marine Worldwide Pacific NE Pacific NE in 2 in in Pacific NE the 336 94 15883 2382 629 Totals worldwide worldwide Totals 422 23420 9621 Totals among orders orders among Totals Salmoniformes P/D 1 1 66 45 17 0.3 2.7 25.8 25.8 2.7 10.5 0.3 0.3 0.1 17 1.4 0.2 45 2 21.5 0.3 43.4 66 1 19 5.4 5 8 273 69 1 52 15 2 Development Order 1271 1 Albuliformes Anguilliformes P Clupeiformes Osmeriformes 1 7 P/D Salmoniformes 72 Stomiiformes 357 42 236 0.8 5 1.5 5.2 33 1.0 Aulopiformes 14.0 P 1.4 Myctophiformes P/D 1 2 5 P 29 3 6 Lampridiformes P/D 13 1 321 Ophiiformes 219 3.7 23 1.4 Gadiformes 10 241 1.6 10 0.9 2 P 7.2 3.5 D 22 1.0 Batrachoidiformes 0.1 3 4.6 4 P P 9.1 6 0.3 Lophiiformes 13 2 355 5 2 1.6Atheriniformes 10 1.5 6.9 482 2.8 Beloniformes 1 146 P/D 19 25 3 2.1 Stephanoberyciformes 5 3.0285 0.3 2 1.2 297 12 5 P/D/V Beryciformes 0.7 3.9 86 1.1 7 1.3 51 Zeiformes 191 1 P/D/V 8 1.6 10 0.4 P 2.4 0.2 1 0.8 2 Gasterosteiformes P 16 11.6 0.5 4 9 P/D/V Scorpaeniformes 19 P/D 1 257 5 123 Perciformes 0.6 4 1.1 1.6 Pleuronectiformes 0.2 1 0.5 P 3 P/D Tetraodontiformes 0.8 11 1 7 570 4 5.1 32 2.4 12 339 5.6 P/D 1922 0.2 1 1.4 9293 143 39.7 3 P/D 22.7 11 39 6 0.3 1 P/D 148 27 P/D/V 1 9 1.5 1 0.2 0.2 2.6 P = pelagic eggs, D = demersal eggs, V = viviparity, Data largely from Breder and Rosen (1966) and Matarese and and (1966) Matarese Rosen Breder from = V eggs, Data viviparity, largely = D eggs, demersal P = pelagic The numbers of families and species worldwide are from Nelson (1994), those which may occur in marine waters of the northeast northeast the of in occur waters marine which may (1994), those Nelson from are and worldwide species families of numbers The 1 Of the worldwide total of 38 orders of teleosts, Pacific. in 23 occur the northeast teleosts, of 38 of total orders worldwide Of the Table 1. Taxonomic distribution of fishes worldwide and in the northeast Pacific northeast the and in worldwide fishes of distribution Taxonomic 1. Table egg of Site 2

Numbers of species Percentofmarine 28 ARTHUR W. K ENDALL, JR.

(PMEL) and biologists from the Alaska Fisheries Science Center (AFSC). The initial paradigm was that recruitment is largely set during the egg and larval stages of pollock by their response to a variety of physical (e.g. advection, turbulence) and biological (e.g. nutrition, predation) processes that determine survival to the juvenile stage. It is thought that surviving larvae pass through a sequence of events during development and that processes which occur during each stage determine the numbers of fish entering the succeeding stage. The hypothesis being tested is that for successful recruitment, transport during the egg and larval stages must be such that most early juveniles reach nursery areas along the Alaska Peninsula. Processes occurring during this transport to juvenile nursery areas are also thought to be important. Each year adult pollock return to the same deep area (>250 m) of Shelikof Strait (Fig. 2) in a remarkably consistent migratory cycle to spawn (Kim and Nunnallee 1990). Although pollock eggs are found throughout the Gulf of Alaska, most adults migrate to Shelikof Strait for spawning, and subsequent development of eggs and larvae occurs there and in downstream waters (Dunn and Matarese 1987; Kim 1989; Kendall and Picquelle 1990). Spawning is con- centrated in late March-early April. Because of the large size of the spawning population, the localized of the spawning area, the short duration of the spawning season, and the lack of strong currents at depth, a large “patch” of eggs is produced that can be recognized through plankton surveys of the region. The eggs quickly rise from their deep hatching depth (150-250 m) to the upper 50 m of the water column (Kendall et al. 1994), where they usually drift to the southwest in the prevailing currents through late April and May. It appears that the larvae usually take one of two routes as they exit Shelikof Strait: along the Alaska Peninsula in the relatively slowly moving coastal waters or over the sea valley in the more rapidly moving Alaska Coastal Current (ACC) (Kim and Kendall 1989). Variations in prey production and distribution may affect pollock larval growth and survival (Incze et al. 1990; Incze and Ainaire 1994; Bailey et al. 1995; Theilacker et al. 1996). Pollock in Shelikof Strait seem to be vulner- able to starvation-induced mortality for the first 2 weeks following the yolk-sac stage, which

160°W 150° 140°

100 m l C. t oasta 60°N i a C a sk la Spawning Str A f Ground o Kodiak I. k li e Gulf of Alaska h ° S 55 a Alaskan l Eggs Stream u s i n n e Kodiak I. e a rv P a a L k s a Al s le ni Semidi ve Is. Ju rly Adult Ea Spawning Migration

Shumagin Is.

Fig. 2. Features of the early life history of walleye pollock in the Shelikof Strait region, Gulf of Alaska. From Kendall et al. 1996. Status of Recruitment Studies ... 29 occurs between the end of April and early May for most larvae. FOCI research has elucidated the dominant current and mesoscale features in the Shelikof Strait region: the ACC, eddies generated by baroclinic instability, and an estuarine-like flow of slope waters into the sea val- ley, and has provided a better understanding of the influence of atmospheric processes on regional oceanography. Pollock larvae are often associated with eddies that are frequently created in lower Shelikof Strait (Schumacher et al. 1993). Apparently larvae are incorporated into eddies as they rise from their hatching depths to surface waters. Since larvae are frequently found in large patches, and the patches have been associated with eddies on several occasions, the question arises as to what advantage this presents to the larvae. In early May 1989, both naupliar concentration (the primary larval food source) and RNA/DNA ratio of larvae were higher in a patch (Fig. 3) indicating that larvae had been feeding more successfully there than larvae outside the patch which could have contributed to decreased growth rates or increased mortality outside the patch (Canino et al. 1991). Current meter records showed the passage of several eddies through the area during spring and summer, and that the currents at the time of the larval study were consistent with the presence of an eddy (Bograd et al. 1994). Later in the year, naupliar abun- dance was greater both in and out of the patch. In addition to providing a good feeding envi- ronment, larvae in eddies may be retained in Shelikof Strait longer than those outside eddies, and thus their probability of transport to nursery areas may be improved. Field data indicate that wind and mixing may also affect larval survival. Bailey and Macklin (1994) suggested several reasons for this relationship, including increased larval prey production in stratified conditions early in the season facilitated by reduced wind-generated turbulence. Laboratory studies (Davis and Olla, in press) and limited field information (Kendall et al. 1994) indicate that pollock lar- vae avoid turbulence above threshold levels, which inhibits swimming and feeding. The presence of relatively warm water initially and the presence of eddies may provide additional favorable conditions for enhanced stratifica- tion and larval survival. In some years (e.g. 1989) conditions seemed favorable for early larval survival, but recruitment was weak, sug- gesting that events later in the early life his- tory of the year class had a negative effect on survival. Larval survival was found to be a necessary, but not sufficient, condition for

Fig. 3. Comparison of in and out of patch conditions. A. RNA/DNA ratios of walleye pollock larvae. Higher ratios indicate larvae in better condition. B. Naupliar concentration (0-40 m). Black bars = mean integrated, and open bars = maximum concentrations. Error bars represent one standard deviation of the mean. From Canino et al. 1991. 30 ARTHUR W. K ENDALL, JR. good recruitment: in some years, events during the age-0 juvenile stage further influence the size of the year class (Bailey et al. 1996). FOCI developed a coupled biophysical model of the dynamics of the early life stages of pollock in Shelikof Strait. It consists of a semispectral primitive equation model (SPEM) of the regional circulation (Hermann and Stabeno 1996) dynamically coupled with an individual-based model (IBM) that follows young fish through the egg, larval and juvenile stages (Hinckley et al. 1997). The trajectories of individual fish through space are computed by treating them as floats, and by using flow fields from the SPEM model. Within the IBM itself, the egg, yolk-sac larval, feeding larval, and juvenile stages are modeled with processes appropriate to each life stage. Prey concentrations are simulated by a Nutrient- -Phytoplankton-Zooplankton (NPZ) submodel which dynamically produces food of the appro- priate size and type for larvae in a spatially explicit manner. The IBM-SPEM model is a valu- able tool to explore the influence of various biological and physical mechanisms on the distri- bution and survival of young pollock in Shelikof Strait. For example, several times and places for spawning were entered into the model to evaluate drift of eggs and larvae (Hinckley et al. in press). Modeled spawning in Shelikof Strait in April resulted in the largest proportion of late larvae reaching the Shumagin Island nursery area. FOCI found several environmental factors that contribute to interannual variation in sur- vival of eggs and larvae, and found that recruitment is largely set during these stages in most years. This work led to development of a conceptual model of the recruitment process of pol- lock spawned in Shelikof Strait (Fig. 4). To evaluate the conceptual model, studies are needed to resolve the mechanisms involved, and also to continue the correlative modeling investiga- tions. FOCI is now providing estimates of relative year-class strength that are considered in management of the Gulf of Alaska pollock fishery. Year-class strength is forecast as weak, average or strong two and three years before the year class recruits to the fishery based on scoring of the biological and physical factors deemed important in the conceptual model (Table 2).

t=0 t=1-14d t=15-21d t=21-59d t=60d-1yr t=1-2yrs t>2yrs

Spaning ol sac eeding One-year Eggs ueniles ecruits dults larae larae olds

Mortality variability little little some most some little little

Mortality/Survival process (proxy) Eddy potential (rainfall) Turbulence (wind mixing) Basin-scale circulation (NEPPI) vigorous sluggish (enhanced prey field) (retain larvae on shelf)

Climate

Fig. 4. Conceptual model of the recruitment process of walleye pollock in the Shelikof Strait region, Gulf of Alaska. From Megrey et al. 1996. Status of Recruitment Studies ... 31

1 Observed forecast forecast Revised Initial forecast orecast/quant.html). orecast/quant.html). Total Puffin diet diet Puffin Larval Larval abundance Advection Advection 2 Mixing 2 Rain Hydro Hydro length Time sequence sequence model model Time series series Time Observed recruitment is based on 20, 40, 60, 80 percentiles of estimated recruitment time series as of September 1999. 1999. September of as series time recruitment estimated of percentiles 80 60, 40, 20, on based is recruitment Observed and rain. mixing of estimates qualitative use forecasts 1995 and previous 1 2 Year Year 1999 Weight 0.00 1998 Weight 0.23 0.00 0.00 1997 Weight 0.00 0.23 0.00 0.00 1996 Weight 0.23 0.21 0.25 0.00 0.00 0.16 1995 Weight 0.25 0.00 0.27 0.30 0.15 0.00 0.25 1994 Weight 0.15 0.20 0.00 0.25 0.50 0.25 0.00 0.27 1993 1.00 Weight 0.25 0.00 0.00 0.13 0.80 0.10 0.00 0.25 1992 1.00 Weight 0.13 0.00 0.00 0.13 0.00 0.25 0.00 0.13 1.00 0.13 1.00 0.00 0.00 0.00 0.13 1.00 0.00 0.00 0.13 0.00 0.00 0.00 0.00 1.00 0.20 0.00 1.00 0.00 0.00 1.00 0.00 1.00 strong=3.0 average-strong=2.5, average=2.0, = weak-average=1.5, 1.0, weak assignments: score Data element 1.0<=weak<1.4<=weak-average<1.8<=average<2.2<=average-strong<2.6<=strong<3.0 continuum: score forecast Recruitment ForecastsTableShelikof 2.of pollock Strait walleye recruitment based researchhttp://www.pmel.noaa.gov/foci/f(from on FOCI Rank Score Total 0.00 0.00 Rank Score s 2.70 Total 0.62 0.00 0.00 Rank 0.00 Score 0.00 0.00 Total 0.00 Element 1.85 a 0.00 0.43 0.00 Rank 0.50 0.60 0.00 0.29 0.21 0.00 0.21 Total Score 0.00 a 0.38 s 2.26 a-s 0.00 2.00 2.19 a Total 0.52 a-s 0.42 a 2.73 0.00 0.68 0.00 Rank 0.84 0.00 0.00 0.29 0.29 0.00 2.00 0.29 Total Score a 0.00 0.32 0.25 2.18 0.00 0.00 1.96 a a-s 0.55 a a-s 0.00 a-s a 2.44 1.67 1.70 0.66 Rank 0.26 1.60 0.00 0.00 0.00 0.00 0.00 1.67 0.00 Total w-a Score 0.00 0.42 0.40 a 2.00 3.00 0.00 2.00 a w-a a 0.30 0.00 a s a w-a 1.72 1.67 0.00 1.00 0.43 0.00 0.00 w-a 3.00 0.00 0.00 0.00 Rank 0.25 0.00 0.00 2.40 0.00 Total 3.00 s w-a w Score 0.65 w 1.67 0.00 2.14 a-s 0.42 0.00 a 2.34 1.98 2.00 0.00 a-s 2.00 a-s Rank 0.20 0.00 2.50 a 2.14 Score 0.63 1.67 0.00 a 1.90 0.00 a-s s 2.34 0.00 1.90 0.00 2.50 a a 0.63 0.00 1.67 a s 2.34 3.00 2.23 0.00 s 0.00 2.23 0.00 3.00 a a-s 0.00 2.34 0.00 2.10 0.00 s 0.00 2.10 2.00 s 0.00 a a 0.00 a 0.00 a 2.00 0.00 w 0.00 2.00 0.00 0.00 2.51 2.14 1.75 32 ARTHUR W. K ENDALL, JR.

Bering Sea Research Pollock dominate the ecosystem of the Bering Sea, providing most of the food for the extensive marine mammal and bird populations found there (e.g. Lowry et al. 1996; Sinclair et al. 1996; Springer and Byrd 1989). In the eastern Bering Sea, there are large interannual variations in recruitment of pollock (more than one order of magnitude), and these drive the population size and thus fishery quotas and harvest levels, and probably have a major impact on other components of the ecosystem as well. To develop an understanding of stock structure and recruitment variation in Bering Sea pollock, the Coastal Ocean Program (COP) of NOAA funded an interdisciplinary seven-year (1991-1997) study, the Bering Sea Fisheries Oceanography Coordinated Investigations (BS FOCI; Schumacher and Kendall 1995). The program goals were to: 1) determine pollock stock structure in the Bering Sea and its potential relationship to physical oceanography, and 2) examine pollock recruitment processes in the eastern Bering Sea. Two of the program’s hypotheses were: 1) interannual variability of larval survival is due, in part, to variability in food resources, and 2) prey densities in the Oceanic Region are generally less than those encountered in the Shelf Region. A major assumption was that springtime production of most larval prey was initiated by the phytoplankton bloom. Pollock in the Bering Sea do not form one homogeneous population, but the actual stock structure is not well known. Within the eastern Bering Sea, there are several spawning areas, and these may also be discrete stocks (Hinckley 1987; Bailey et al. 1999). These spawnings result in a broad geographic distribution of pollock eggs (Fig. 5). Pollock spawn mainly in two distinct regions in the southeast Bering Sea: the Oceanic Region and the Shelf Region. Spring phytoplankton bloom dynamics vary between the two regions as a result of dissimilarities in their physical characteristics. This leads to differences in conditions that affect the survival of larvae emerging in the two regions (Napp et al. 2000). In the Oceanic Region, advection is strong, but variable, and mesoscale features (eddies) are common. Conditions outside mesoscale features show low interannual variability. Variabil- ity in observed plankton standing stocks may be largely due to advective rather than local processes. Larval prey concentrations in the Oceanic Region are generally low and are domi- nated by less preferable prey for pollock larvae. This may necessitate the larvae supplementing their diet with protists. Mesoscale features may contain much higher prey densities with a larger proportion of the prey types preferred by first-feeding larvae. In some years the Bering Slope Current is eddy-rich, and pollock larvae are found in association with these eddies (Schumacher and Stabeno 1994). To the extent that phytoplankton biomass is a proxy for larval prey concentrations, larvae that find themselves in these dynamic features may have a lower probability of starvation than those outside such features. In the Shelf Region, advection is generally low, but there is considerable interannual variability from a complex set of forcing functions: sea ice, wind mixing, temperature, and insolation result in phytoplankton blooms with different timing, magnitude, and transfer effi- ciencies to the zooplankton and nekton (or benthos). Sea ice affects the stability and tempera- ture of the water column, the developmental rate of larvae, and the timing of the phytoplankton bloom (and potentially the match/mismatch of larvae and prey). Ice cover and the cold pool also influence distributions of higher trophic level biota (Ohtani and Azumaya 1995; Wyllie- Echeverria 1995). In the Shelf Region, preferred prey have been found to be a larger fraction of the available prey than in the Oceanic Region, but copepod nauplii were still at minimal levels for growth and survival (Napp et al. 2000). Status of Recruitment Studies ... 33

Fig. 5. Historical distribution of maximum densities of walleye pollock eggs in the southeastern Bering Sea (Dell Arciprete, pers. comm.). Data are from 15 cruises, February to May, 1971-1993. Sampling quadrants are the dark squares and the black dots are where eggs were found. From Napp et al. 2000.

Formation of sea ice in the eastern Bering Sea generally begins in November with maxi- mum ice extent occurring in late March. A bloom of phytoplankton associated with the sea ice (Stabeno et al. 1998) accounts for 10 - 65 % of the total annual primary production (Niebauer et al. 1990). Previously unreported under-ice spring phytoplankton bloom dynamics were observed using moored biophysical instruments (Fig. 6). In 1995, a heavy ice year, the under- ice bloom persisted despite an unstable water column: Lagrangian drifters with optical sensors did not detect a subsequent main bloom. Spring winds that year were not strong enough to 34 ARTHUR W. K ENDALL, JR.

Fig. 6. Time course of a spring phytoplankton bloom under sea ice on the southeastern Bering Sea shelf during a heavy ice year (1995). Upper panel shows temperature with depth, lower panel shows period of ice cover and chlorophyll concentration (mg · m-3) at 7 and 44 m. Modified from Stabeno et al. 1998. erode the pycnocline and replenish surface nutrients. In 1996, a year with minimal ice, the spring bloom was not detected until the first week of May. Thus, the presence or absence of sea ice and springtime winds has an influence on the timing, location, and number of spring blooms. Variability between these two physical factors (ice and wind) may have a strong impact on larval pollock survival in the spring. Thus, it is believed that the key to interannual variation in food for first feeding pollock larvae in the Oceanic Region is advection, while in the Shelf Region it is the coupled dynamics of the atmosphere-ice-ocean system. While the BS FOCI was unable to relate feeding success of early larvae directly to year-class success of Bering Sea pollock, a conceptual model of this portion of the life history was developed (Fig. 7), which has provided guidance for future studies. Partially as a follow up to the BS FOCI, NOAA’s COP has funded a further multi-year (1996-2000) study, the Southeast Bering Sea Carrying Capacity (SEBSCC) program looking more broadly at the role of pollock in the Bering Sea ecosystem. Emphasis is on the juvenile stage of pollock; at this time they are preyed upon by a variety of predators including adult pollock, groundfish, marine mammals and seabirds. They are abundant enough to compete for food (zooplankton) with other components of the pelagic ecosystem. There is evidence that cannibalism may be a major source of mortality of juvenile pollock, and interannual differ- ences in the overlap of juveniles and adults may influence year-class success (Bailey 1989). SEBSCC is examining oceanographic and biological factors that may influence the productiv- Status of Recruitment Studies ... 35

Effective Prey Concentration

Advection & Mesoscale Variability Oceanic Region

Spawning Yolk sac Feeding Eggs Adults larvae larvae

Presence or Absence of Sea-Ice & Water Temperature

Wind Mixing/Turbulence

Shelf

Region Timing of Preferred Prey Production

Fig. 7. Conceptual switch model of walleye pollock early larval dynamics for the Oceanic and Shelf Regions of the southeastern Bering Sea. From Napp et al. 2000. ity of the area and the distribution and abundance of juvenile pollock. It appears that SEBSCC field studies are observing major ecosystem changes in the Bering Sea (Macklin 1999). Un- usually warm surface water occurred in the summers of 1997 and 1998 and large blooms of coccolithophorids were seen for the first time in the Bering Sea (Vance et al. 1998). The causes and impacts of these blooms are not yet fully understood, but there were concomitant major seabird die-offs and lower-than-expected salmon runs (Kruse 1998). The influence of these unusual conditions on pollock recruitment is the subject of ongoing research (Napp and Hunt, accepted).

FUTURE DIRECTIONS

Time scales of variability and biological responses While most recruitment research to date has focused on interannual variations in year-class strength of individual species or stocks, it is becoming increasingly apparent that inter-species and inter-year patterns of recruitment exist. For examples consider: sardine and anchovy recruitment varies over a cycle of about 60 years in eastern boundary currents around the world (Baumgartner et al. 1992; Klyashotorin and Smirnov 1995), the gadoid outburst in the North Sea (Cushing 1980), shifts in dominance between and mackerel in the northwest Atlantic (Skud 1982), a correlation between recruitment of cod and pollock in the eastern Bering Sea, and the inverse relationship between production of several Pacific salmon stocks in Alaska waters and those off the west coast of the United States (Mantua et al. 1997). Besides interannual 36 ARTHUR W. K ENDALL, JR. variability and local conditions, recruitment is likely affected by environmental processes that vary on large spatial and low frequency temporal scales. Future recruitment studies should not ignore these environmental processes and how they affect recruitment. Significant advances have been made in the last decade in understanding climate varia- tion and its impact on the ocean. The atmosphere varies on several time scales from diel, to event (storm), to seasonal, to interannual, to decadal, and longer (global climate change). The importance of decadal variability on marine communities and productivity is just now being recognized (Francis et al. 1998). Decadal variability is due to a number of processes that can act together or oppose each other to produce conditions observed at any one time. Extreme conditions occur when all of these processes push the climate in the same direction. In the north Pacific, the El Niño-Southern Oscillation is the best understood large-scale process that affects the climate and ocean. El Niños are now being forecast and their influence on the weather and physical ocean conditions are predicted (Chen et al. 1995). They impact ocean productivity through disturbance of coastal upwelling. Distributions of marine organisms are changed due to warming of the water. El Niños seldom last more than a few months and occur once or twice a decade. Although El Niños originate in the tropics, the effects of some of them on the ocean can be measured at high latitudes. These effects reach high latitudes both through direct transfer of energy and through teleconnections between atmospheric conditions in the tropics and at higher latitudes. The next longer scale of climatic variation in the north Pacific is characterized by the Pacific Decadal Oscillation (PDO)(Mantua et al. 1997). The phase of the PDO changes at frequencies of about 20-30 years. Negative PDO values are associated with relatively warm sea surface temperatures in the northeast Pacific. PDO was negative through most of the 1960s and 1970s and has been mainly positive since 1977. The abrupt change in magnitude and sign of the PDO between 1976 and 1977 (Miller et al. 1994) coincided with dramatic changes in population levels of numerous species in the northeast Pacific and has been termed a Regime Shift (Beamish et al. 1999). The entire ecosystem of the region may have shifted from one steady state condition to another at this time. Enhanced primary productivity caused by higher temperatures and shallow mixed layer depths following the 1977 regime shift may have worked its way up the food chain through increased zooplankton abundance (Brodeur and Ware 1992) to produce the high abundances of Alaska salmon that have been present since then. At an even longer time scale, current increasing temperatures may signal global climate change. Schumacher et al. (submitted) have reported recent increases in temperature and de- creases in sea ice in the Bering Sea, and Wiles et al. (1998) have reported recent warming in the Gulf of Alaska. Several groups of north Pacific and Bering Sea marine animals are showing long-term monotonic changes in abundance (e.g. Roemmich and McGowan 1995; Hill and Demaster 1988; Brodeur et al. 1999). Whether these are responses to conditions associated with gradual large-scale increases in temperature, and whether these changes are partially or wholly man-induced are subjects of considerable ongoing research.

A new paradigm for recruitment research Given that lower frequency (longer than interannual) variations in recruitment are commonplace, and may be the most important mode in determining abundances of many stocks, and that recruitment patterns of co-occurring species are often inter-related, how should we study Status of Recruitment Studies ... 37 recruitment? Statistical procedures examining patterns of recruitment in relation to environmental variables may point to areas of importance, but they are unlikely to produce an understanding of biological responses to the environment. They will be no more effective for understanding or predicting recruitment than stock/recruitment based studies have been in the past. An understanding will involve examining mortality/survival of eggs, larvae and juveniles: what are the age-specific rates, how do they vary, and what are the causes of variation? We should try to pry open the black box of recruitment since this holds the key to longer term trends (Steele 1996). Thinking that one factor acting on one life stage is responsible for recruitment variation will continue to be unsuccessful. Even for one species, the factors and stages that are critical may vary with time. Therefore, I propose that to understand recruitment well enough to pro- duce forecasts that managers need, research must consider year-class variation in light of longer time scales and ecosystem processes. Time scales: Single season or short-term projects will not provide the answers. Recruit- ment in many species is autocorrelated: year-class strength varies on time scales longer than interannual. Studies of even five years duration may only observe one general level of recruit- ment, or at best the transition from one level to another. While there are interannual variations in the environment, these occur in the context of longer scales of variability. Climatic events such as regime shifts may create perturbed ecosystems that take several years to regain a state of maturity, where biological processes are more important than physical processes in deter- mining population levels. There are variable lags between changes in abundance of long-lived predators and their prey in their response to low frequency environmental conditions. Recruit- ment studies must take into account the climatic state of each year under investigation. Clearly, long time scales (20-40 years) must be considered. Retrospective, laboratory and modeling studies can partially augment field studies to extend the time horizon. Multiple trophic levels: The importance of both bottom-up (food) and top-down (preda- tion) processes probably varies among species and stages within a species, and may change under different environmental conditions. The basic productivity (carrying capacity) of ocean systems seems to change under different climatic regimes (e.g. Roemmich and McGowan 1995; Polovina et al. 1994). Interannual and longer time scale temperature differences may impact the timing of the spring bloom and rate of reproduction and development of copepods which provide food for larvae and cause a mis-match between food availability and occurrence of larvae. The abundance and distribution of larval and juvenile predators may likewise be im- pacted by large-scale atmospheric and oceanic conditions, and thus influence survival. Also, the effects of parasites and diseases cannot be ruled out as sources of mortality. Therefore, recruitment studies must examine factors affecting production and distribution of trophic lev- els both above and below the early stages of fishes. Multi-species focus: Since recruitment of several species in an area often seems to be related either positively or negatively, more understanding should be gained by studying them together. In fact, the recruitment process of one such species may never be understood without taking into consideration co-occurring species. In some cases, different temperature regimes seem to favor one species over another, but what is the actual causal mechanism? For example, is it development rate of the eggs and larvae, or does temperature affect the production of larval food: timing and preferred species? Once a species becomes dominant, can it prevent another species from assuming that role through predation or competition? 38 ARTHUR W. K ENDALL, JR.

CONCLUSIONS

Recruitment variation was a major focus of fisheries research through most of the twentieth century, but year-class strength continued to be as unpredictable as it was in Hjort’s time at the beginning of the century. With the beginning of a new century, and its promise of better tools to apply to the problem, should we expect to increase our understanding? I think the answer is yes, but only if we look at the problem from a much broader perspective than we have in the past. Will we have a chance to do so? Only our funding agencies can answer that. “I continued to work intermittently on the Stock Recruitment Relationship. We now know that the only way in which the problem will be solved is in the examination of the dynamics of life in the larval stages. The research is needed and, without it, management will remain self-defeating. Paymasters will learn this, if slowly, so slowly” (Cushing 1996).

REFERENCES

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